There are four yugas: Satya-yuga, Tretā-yuga, Dvāpara-yuga and Kali-yuga. So the aggregate of all these yugas is about forty-three hundred thousands of years. And if you multiply by one thousand, then that becomes Brahmā's one day only, twelve hours. Sahasra-yuga-paryantam arhad yad brahmaṇo viduḥ. So similarly, he has got one night also, and similarly, one day and night, then one month, then one year. In this way he lives for one hundred years. So during this period of daytime, Kṛṣṇa appears once. That we learn from the Vedic literature.
So Kṛṣṇa said that what is His mission to come here. Kṛṣṇa is God. He does not require to come here, but He comes to serve some purpose. If God wants to come, nobody can check. Sometimes people question that, "Why God should come?" So our answer is, "Why God should not come?" If He is all-powerful, who can check Him to come here? If we say that God cannot come, that means God becomes under our rules and regulation. So according to Vedic scripture, God comes, and He says personally why He comes: yadā yadā hi dharmasya glānir bhavati (BG 4.7). Whenever there is discrepancies in the prosecution of religious principles, He comes. Yadā yadā hi dharmasya glānir bhavati, abhyutthānam adharmasya. And whenever there is discrepancies in religious procedure, irreligious activities increase. That is natural. Whenever there is lenient government, the rogues and thieves will increase. It is natural. And if the government is very strict, then rogues and thieves cannot become very prominent. So when Kṛṣṇa comes, He has got two business: paritrāṇāya sādhūnāṁ vināśāya ca duṣkṛtām (BG 4.8)—for giving protection to the devotees, to the faithful, and for killing the demons.